1. Field of the Invention
The present invention relates to an apparatus and method for photoacoustic imaging, and in particular, relates to a technique for measuring optical characteristics of the interior of a specimen using the photoacoustic effect.
2. Description of the Related Art
One of photoacoustic imaging techniques is photoacoustic tomography (PAT). According to the PAT technique, a specimen, such as biological tissue, is irradiated with pulsed light emitted from a light source. A light absorber absorbs the light propagated and scattered in the specimen, thus generating an acoustic wave. Such an acoustic wave generating mechanism is called the photoacoustic effect. A light absorber, such as tumor, often has a higher light energy absorption coefficient than its peripheral tissue. Accordingly, the light absorber absorbs more light than the peripheral tissue and instantaneously expands. In a photoacoustic imaging apparatus utilizing the photoacoustic effect, acoustic wave detecting devices receive an acoustic wave generated upon expansion, thus obtaining received signals. The received signals are mathematically analyzed, so that information about, for example, a distribution of sound pressure of the acoustic wave generated in the specimen can be imaged. A distribution of optical characteristic, particularly, absorption coefficient in the specimen can be obtained on the basis of image data obtained in this manner.
In PA imaging, light applied to a specimen may cause an acoustic wave (interfacial acoustic wave) on the surface (hereinafter, also referred to as “interface”) of the specimen. The details will be described later. When the interfacial acoustic wave is received by an acoustic wave detecting device, a signal output from the acoustic wave detecting device includes a transient response caused by the limitation of a receivable frequency band of the detecting device. This transient response appears as an artifact in an image obtained by PA imaging. The artifact is an image which does not really exist but appears as if something exists there and is also called a ghost. If an acoustic wave caused by a light absorber reaches the acoustic wave detecting device after the interfacial acoustic wave, an image of the light absorber, such as tumor, may be hidden by the artifact. Alternatively, if a holding member, such as a plate, for fixing or holding a specimen is used upon acoustic wave measurement, an interfacial acoustic wave is reflected multiple times inside the holding member. Such a reflected wave (reflected interfacial acoustic wave) is also detected by the acoustic wave detecting device. Thus, the reflected interfacial acoustic wave also causes a transient response similar to the above described one. Disadvantageously, an image of a light absorber may be hidden by an artifact caused by the transient response.
The following problem is similar to the above-described problem caused by the reflected interfacial acoustic wave in the photoacoustic imaging apparatus. In an ultrasonic measuring apparatus using an ultrasonic echo, multiple reflections of a transmitted ultrasonic wave are repeated inside a member interposed between an acoustic wave detecting device and a specimen, thus causing artifacts. The artifacts caused by multiple echoes appear in an image.
A method of eliminating such artifacts caused by multiple echoes is disclosed in Japanese Patent Laid-Open No. 2000-107177. According to the method disclosed in Japanese Patent Laid-Open No. 2000-107177, an average signal obtained by averaging a plurality of received signals is subtracted from a received signal, thus eliminating an amplitude caused by multiple echoes.
Applied light generally has a spatial distribution of intensity. Thus, there is a difference in light intensity at different positions. The amplitude of an acoustic wave caused by the photoacoustic effect is proportional to the light intensity distribution. Accordingly, the above-described interfacial acoustic wave has a spatial distribution of sound pressure proportional to the spatial distribution of intensity of light applied to the interface of a specimen. Similarly, the reflected interfacial acoustic wave has a spatial distribution of sound pressure. The feature in which the spatial distribution of sound pressure is uneven is peculiar to the photoacoustic imaging apparatus having a feature in which the spatial distribution of light intensity is uneven.
In the photoacoustic imaging apparatus, an acoustic wave is received in different positions. Simultaneous reception of the acoustic wave in the different positions can reduce measurement time. Accordingly, a device array including acoustic wave detecting devices arranged one-dimensionally or two-dimensionally is generally used. Since the acoustic wave detecting devices receive an interfacial acoustic wave whose spatial distribution of sound pressure is uneven, the amplitude of the interfacial acoustic wave received differs from detecting device to detecting device. If the method disclosed in Japanese Patent Laid-Open No. 2000-107177 is used, a plurality of received signals having different amplitudes are averaged. An averaged amplitude caused by multiple echoes does not necessarily match an amplitude caused by multiple echoes in each received signal. Disadvantageously, if an average signal is subtracted from the received signal, the amplitude caused by multiple echoes is not sufficiently reduced in some cases. In other words, the method disclosed in Japanese Patent Laid-Open No. 2000-107177 is effective in the ultrasonic measuring apparatus in which a received signal amplitude caused by multiple echoes is constant. If the method is applied to the photoacoustic imaging apparatus in which the spatial distribution of sound pressure is uneven, it is difficult to achieve a satisfactory effect.